1. Field of the Invention
This invention relates to a method for the preparation of fully dense, consolidated diamond or diamond composite articles as for use in cutting and drilling applications. The invention involves heat treating the article to improve wear and fracture resistance.
2. Description of the Prior Art
The conventional synthesis of diamond grit or powder involves the conversion of a non-diamond carbon to diamond in the presence of a metal acting as a solvent-catalyst under conditions of high temperature and high pressure at which diamond is the thermodynamically stable form of carbon. Although various carbonaceous materials, such as charcoal, coal, coke and graphite may be used as the carbon source, typically graphite, and specifically spectrally pure graphite, is used almost exclusively for the commercial production of diamond grit or powder. It is also possible to use as the carbon source carbon-containing organic compounds, such as anthracene, fluorene, pyrene, sucrose, camphor and the like.
With the use of graphite as the carbon source in accordance with the preferred practice, the graphite may be a powder, a disc of compressed and machined powder or a capsule into which the graphite is placed with the solvent-catalyst. Typically, the graphite charge in one of the above forms is converted to diamond in the presence of one or more metals or metal-containing compounds serving as the solvent-catalyst. Graphite can be converted directly to diamond in the absence of a catalyst but pressures of about 130 kbar and temperatures in the range of 3000 to 4000.degree. C. are required. With the addition of conventional solvent-catalysts lower pressures and temperatures may be used in the range of 1200 to 1600.degree. C. and 50 to 80 kbar, respectively. The solvent catalyst dissolves the graphite until a saturated solution of carbon relative to graphite is obtained. The catalytic effect is the promotion of the structural rearrangement of the graphite to diamond. Significant solvent-catalysts are the Group VIII transition metals, including platinum, chromium, tantalum, manganese and alloys containing at least one of these metals. Iron, nickel, cobalt and manganese are the preferred pure metal solvent-catalysts and iron-manganese, nickel-chromium, nickel-manganese, iron-nickel, nickel-cobalt and various other nickel-containing alloys are generally the preferred alloys for this purpose. The solvent-catalyst is employed either in powder form, which may be loose or compacted, or in the form of a disc.
Other additives, such as boron, are sometimes used as additions to the charge to be compacted in order to change one or more of the properties of the resultant diamond consolidated article. The solvent-catalyst to carbon volume ratios are typically 0.1 to 10 with a preferred ratio of 0.5 to 2. Consolidation is achieved typically by the use of belt presses or cubic presses. The constituents are loaded into a cell of cylindrical configuration. Heating is usually provided by passing an electric current directly through the charge within the cell.
The graphite-to-diamond conversion is performed in the diamond stable region of temperatures and pressures in the range of 1200 to 2500.degree. C. and 50 to 120 kbar, respectively. The reaction time is usually within the range of 0.5 to 20 minutes.
Broadly, the consolidation sequence includes pressurization, heating to desired maximum reaction temperature, reaction time to permit conversion, cooling down and pressure release.
After cooling of the consolidated charge or mass, the diamond crystals are separated from the metal matrix by acid dissolution. For this purpose, nitric acid may be used at a temperature of 100 to 300.degree. C., which dissolves all of the constituents except the diamonds. The diamonds may then be separated from the liquid by centrifuge or filtration. If, after this dissolution step, the diamonds are agglomerated, they may be separated by a light-crushing operation. The separated diamonds may then be sorted according to size and shape. The size of the resulting diamond grit or powder may vary within the range of 1 micron up to about 1 mm.
Diamond particles so produced may be compacted into a substantially fully dense consolidated article, such as drill blanks for use in producing drill bits. For this purpose, the charge may be compacted to produce a monolithic structure or may be compacted onto a disc or substrate of for example tungsten carbide and cobalt. The resulting disc of the substrate with a diamond layer thereon may be assembled in various configurations depending upon the cutting or drilling device with which it is assembled. In any event, consolidation is achieved by sintering the diamond powder at high temperatures and high pressures in the diamond stable region in the presence of a catalyst or a non-catalytic sintering aid to obtain a strong, interbonded, polycrystalline consolidated mass or article of substantially full density. The apparatus used for compacting may be the same as that used in the synthesis of the diamond particles. Cell assemblies typically used in these applications are described in U.S. Pat. No. 3,407,455 and U.S. Pat. No. 4,604,106.
According to prior-art compacting or consolidating practice, including the U.S. patents listed hereafter, the preferred charge is diamond powder although graphite powder may be mixed therewith. In this application, the diamond powder generally constitutes at least 70 volume % of the total mass, preferably 90 to 99%. The final compacted article has diamond grains of 10 to 20 microns but depending upon the temperature may have a large-grain structure of about 100 microns.
Prior to charging of the diamond powder to the cell, the diamond powder may be cleaned by heating it in the presence of hydrogen gas typically for one hour at a temperature within the range of 800 to 1000.degree. C. Boron may be employed as a sintering aid for the diamond powder and is typically introduced by doping the diamond powder prior to introducing the diamond charge to the cell for compacting. Also, a pretreatment step involving surface graphitization of the diamond powder may be performed to provide thereon a uniform coating of graphite which promotes the penetration of the catalyst into the diamond layer within the cell by continuously dissolving the graphite to form diamond during high temperature compacting. The catalyst-carbide charge may consist of cobalt, nickel or iron catalyst powder mixed with tungsten carbide, titanium carbide or tantalum carbide powder.
The following table lists patents representative of conventional practices relating to consolidated diamond or diamond composite articles:
______________________________________ Issue U.S. Pat. No. Date Summary ______________________________________ U.S. Pat. No. 3,141,746 7/21/64 Diamond compact abrasive by sintering a mixture of diamond powder (50+ vol %) with a catalytic metal powder (one or more of Fe, Ni, Co and Ti) in the diamond stable region. U.S. Pat. No. 3,574,580 4/13/71 A method of making interbonded diamond compacts by sintering clean diamond powder in the diamond stable region, optionally intermixed with up to 3 wt % B, Si, or Be powder as a sintering aid. U.S. Pat. No. 3,745,623 7/17/73 Relates to powder diamond compact blanks having a 70+ vol % interbonded diamond layer joined to a cemented carbide substrate and the method for making them. U.S. Pat. No. 4,224,380 9/23/80 A temperature-resistant, bleached powder diamond compact formed by the removal of the metallic catalyst phase from the interbonded diamond. U.S. Pat. No. 4,288,248 9/8/81 A method of making the temperature-resistant, leached powder diamond material and compacts of U.S. Pat. No. 4,224,380 by acid leaching. U.S. Pat. No. 4,518,659 5/21/85 An improved process for making powder diamond compacts using a first catalyst (copper) to sweep through the diamond charge preceding a Co catalyst. U.S. Pat. No. 4,592,433 6/3/86 A powder diamond compact blank of diamond strips in a grooved cemented carbide substrate. ______________________________________ As described in U.S. Pat. No. 3,745,623, the cell assembly for consolidation may include a salt spacer, a zirconium disc separator, a tungsten carbide/cobalt disc, a diamond powder layer and an additional zirconium disc separator. The loading sequence involves the stacking of several single charges of this construction in a zirconium or tantalum metal sheath or capsule which is placed in the cell after the capsule is full.
The steps involved in consolidation are similar to the processing for synthesis of the diamond powder. Namely, the process includes pressurization from 0.001 to 50 kbar or greater and heating up to sintering temperature from 20 to 1500.degree. C. or greater. Sintering may be effected at a pressure of 50 kbar and a temperature of 1500.degree. C. for 10 minutes. Cooling down is then effected from a temperature of 1500.degree. C. to a temperature of 20.degree. C. or less with pressure release from 50 to less than 0.001 kbar. Sintering is generally conducted within the temperature range of 1200 to 1600.degree. C. and the pressure range of 40 to 70 kbar. Sintering times are generally within the range of 10 to 15 minutes, particularly when a belt press is employed, with sintering times less than 3 minutes being possible with the use of a cubic press.
After consolidation, the stack of sintered, consolidated blanks is separated manually and the zirconium capsule is removed. Lapping and polishing operations are employed to remove any particles of material adhering to the edges and flat surfaces of the blanks. The blanks are then ground to shape, which is typically cylindrical, and to the dimensions required for the particular cutting or drilling assembly with which they are to be used. Sizes conventionally employed for this purpose are diameters of about 1 to 5.5 cm with thicknesses of 3.5 to 8 mm, which includes a 0.7 mm to 1 mm thick diamond layer on the blank assembly. The resulting product is a disc of two layer structure, specifically a substrate of a composition, such as tungsten carbide and cobalt, with a fully dense layer of diamond particles bonded thereto.
Further post-consolidation acid leaching treatments involving nitric and hydrochloric acid have also been used for fabricating temperature-resistant powder diamond material and compacts, from which most of the interpenetrating network of Co has been removed. This prior art is described in U.S. Pat. No. 4,224,380 and U.S. Pat. No. 4,288,248.
For specific applications such as the production of drill bits used in oil well drilling applications, the disc is mounted on a cutter by the use of a brazing step. Specifically, the blank is brazed onto a tungsten carbide/cobalt post. A plurality of these post-disc assemblies are then mounted on drill bits of various configurations with the diamond portion of each acting as a cutting surface. Multiple cutters of leached powder diamond material may also be brazed into the surface of matrix-body drill bits, replacing either some or all of natural diamond stones typically used in such bits. Drill bits of these types constructions are well known in the art.
During use of the above-described consolidated diamond articles for cutting and drilling applications, it is advantageous that these articles be characterized by high wear resistance and resistance to cracking. Applicants have determined, in this regard, that during the high-temperature compacting operation to achieve the fully dense, consolidated diamond article, dislocations in the diamond crystal structure result. Dislocations in crystalline materials, such as diamond, are linear regions of lattice imperfection. These imperfections allow the crystal to undergo plastic deformation at sufficiently high temperatures and are also generated by the deformation process. Since these imperfections consist of regions of lattice distortion, they generate highly localized stress fields. As such, they provide sites for crack initiation and propagation when the diamond article is under high applied stress characteristic of use thereof in cutting and drilling applications. Likewise, these dislocations in the diamond crystal structure adversely affect the wear resistance of the consolidated diamond article during use thereof in cutting or drilling applications by providing sites for fracture or chipping away of the article at regions of stress difference caused by these dislocations. Applicants have determined, therefore, that these dislocations resulting during the high temperature compacting of diamond particles to form a consolidated article therefrom adversely affect the performance of the article from the standpoints of both cracking, which in severe instances may result in catastrophic failure, and wear resistance.